MDL-41451 - Large forms are truncated by max_input_vars

By parsing php://input in chunks, we can bypass max_input_vars for
forms which do not use the "multipart/form-data" enctype.  This can
be (and is) safely skipped if there are fewer than max_input_vars
fields submitted as part of $_POST.

lib/formslib.php

@@ -261,10 +261,10 @@ abstract class moodleform {
         $submission = array();
         if ($method == 'post') {
             if (!empty($_POST)) {
-                $submission = $_POST;
+                $submission = $this->_get_post_params();
             }
         } else {
-            $submission = array_merge_recursive($_GET, $_POST); // emulate handling of parameters in xxxx_param()
+            $submission = array_merge_recursive($_GET, $this->_get_post_params()); // Emulate handling of parameters in xxxx_param().
         }
 
         // following trick is needed to enable proper sesskey checks when using GET forms
@@ -282,6 +282,38 @@ abstract class moodleform {
         $this->_form->updateSubmission($submission, $files);
     }
 
+    /**
+     * Internal method. Gets all POST variables, bypassing max_input_vars limit if needed.
+     *
+     * @return array All POST variables as an array, in the same format as $_POST.
+     */
+    protected function _get_post_params() {
+        $enctype = $this->_form->getAttribute('enctype');
+        $max = (int)ini_get('max_input_vars');
+
+        if (empty($max) || count($_POST, COUNT_RECURSIVE) < $max || (!empty($enctype) && $enctype == 'multipart/form-data')) {
+            return $_POST;
+        }
+
+        // Large POST request with enctype supported by php://input.
+        // Parse php://input in chunks to bypass max_input_vars limit, which also applies to parse_str().
+        $allvalues = array();
+        $values = array();
+        $str = file_get_contents("php://input");
+        $delim = '&';
+
+        $chunks = array_map(function($p) use ($delim) {
+                return implode($delim, $p);
+            }, array_chunk(explode($delim, $str), $max));
+
+        foreach ($chunks as $chunk) {
+            parse_str($chunk, $values);
+            $allvalues = array_merge_recursive($allvalues, $values);
+        }
+
+        return $allvalues;
+    }
+
     /**
      * Internal method. Validates all old-style deprecated uploaded files.
      * The new way is to upload files via repository api.